1. Field of the Invention
The present invention relates to a method of switching communications in a mobile communication system, more particularly relates to a handover method for switching base transceiver stations and access points in accordance with movement of a mobile terminal.
2. Description of the Related Art
In recent years, in addition to the top communication media of W-CDMA and other cellular networks, other communication media such as wireless local area networks (LAN's) and the Personal Handyphone System (PHS) network have become usable at relatively low charges. Therefore, it has become possible to communicate while freely selecting among these communication media in accordance with the signal conditions at the location or the communication conditions required by the user.
“Mobile IP” is the latest specification of TCP/IP considering movement of terminals. It is also best suited for use in communication environments enabling a plurality of types of communication media to be selected from. Notebook personal computers (PC's), personal digital assistants (PDA's), mobile phones, PHS terminals, etc. designed for the Mobile IP are rapidly spreading in use.
The present invention proposes a new technique for the handover essential when a notebook PC, PDA, or other mobile terminal based on Mobile IP technology switches base transceiver stations or access points along with movement in a communications environment including a plurality of such communication media. Note that as known art relating to the present invention, there are Japanese Unexamined Patent Publication (Kokai) No. 2004-200789 and Japanese Unexamined Patent Publication (Kokai) No. 2004-200789 discloses a system particularly able to prevent an increase in network traffic when there are insufficient management resources of a home agent in a Mobile IP network environment, while provides a “seamless link” as a product using the Mobile IP standard.
As will be explained in detail later with reference to FIG. 10 and FIG. 11, the above “Mobile IP” standard basically calls for registering the correspondence between home addresses (HoA) and “care-of” addresses (CoA) in a home agent (HA). Here, a “home address” (HoA) is an address assigned to each mobile terminal and remains unchanged even with movement. Further, a care-of address is an address assigned to each communication media (network card) which a mobile terminal can utilize at a destination and changes along with movement. By registering pairs of fixed home addresses (HoA) and variable care-of addresses (CoA) in the home agent (HA), a terminal communicating with a registered mobile terminal can continue communicating with that mobile terminal automatically regardless of the latter's location, no matter where that mobile terminal moves to, that is, no matter how the care-of address (CoA) changes, so long as connected to the home address (HoA).
However, according to the handover method using the known Mobile IP technology, the following problem arises if switching communication among different communication media (communication networks).
There is the problem that at the time of the above switching of communication, the transmission packets in the middle of transfer are lost, i.e., so-called “packet loss” ends up occurring. This problem naturally causes degradation of the quality of service (QoS) and causes obstacles when real time communication is required such as with IP telephoning.
This problem becomes particularly conspicuous in handover during communication using the recently fast spreading wireless LAN's. For example, the above degradation of the quality of service becomes remarkable under circumstances where the IP address (CoA) is changed for handover at the time when a mobile terminal shifts from connection with the communication network of a current access point (AP) to connection with a communication network of a separate access point (AP) along with movement of a user. This is due to the following reason.
That is, when switching the communications network from a current access point (AP1) to another access point (AP2) (handover), it is necessary to go through the later explained authentication process and care-of address (CoA) assignment process, so a relatively long suspension of service of for example several seconds ends up occurring. In the end, it is only after several seconds have past from the start of switching of the communications network that registration at the new CoA by the Mobile IP finishes. During the suspension of service, the above packet loss and therefore degradation of the quality of communication end up occurring.